1. Field of the Invention
This invention relates to nuclear chlorination of aromatic compounds having an electron donating group, particularly phenols and aromatic amines.
2. Description of the Prior Art
The nuclear chlorination of aromatic compounds having an electron donating group is known. For example, the chlorination of phenols by cupric chloride in hot aqueous hydrochloric acid is disclosed in Japanese Pat. publication No. Sho 45-40882 (40882/70), dated Dec. 22, 1970. The process disclosed in this publication is advantageous in comparison with prior processes because the ratio of the desired para-substituted product to the ortho-substituted product is increased and less severe reaction conditions are required.
Additional descriptions of this process and related processes are set forth in (1) H. P. Crocker and R. Walser, J. Chem. Soc. (c), 1970, 1982-1986, (2) H. P. Crocker and R. Walser, Chem. and Ind., 1969, 1141-1142, and (c) German Offen. Nos. 1800676, 1926852 and 2014773.
In the process of the above-referenced Japanese patent publication, for economic reasons, after the chlorination reaction is completed, the solution of copper chlorides dissolved in aqueous hydrochloric acid solution (hereinafter referred to as the "working solution") is separated from the organic products. The working solution is then oxidized with a molecular oxygen-containing gas and hydrogen chloride gas to convert the cuprous chloride therein to cupric chloride and the working solution is then recycled for use in the next chlorination reaction. This is a batchwise recycling method. It is also proposed to carry out a continuous method in which the chlorination of the aromatic compound and the oxidation of cuprous chloride to cupric chloride are carried simultaneously by introducing hydrogen chloride gas and molecular oxygen-containing gas into the chlorination reaction solution.
There are some disadvantages in the above-mentioned processes. In the case of the above-mentioned batchwise recycling method, the concentration of cupric chloride in the chlorination reaction solution is reduced as the chlorination reaction progresses and the rate of the chlorination reaction falls remarkably. To keep the conversion of phenols at about 80-90%, it is necessary to chlorinate for a long time or to use a large excess of cupric chloride, such as 3-5 times the theoretical amount. It is evident that such a method has numerous disadvantages, including increased cost of equipment because of the additional reaction vessel required for regenerating the working solution, increased cost of catalyst, reduction of the manufacturing capacity because of the additional regeneration process, and the cost of expensive reaction and regeneration materials.
In the case of the continuous method, some advantages are achieved such as lowering of the catalyst cost, shortening of the reaction time because of a higher reaction rate and obtaining a good conversion with less cupric chloride than is the case with the recycle method. But the continuous method does not avoid other disadvantages such as increased cost of equipment and the use of expensive materials, and the need for expensive safety precautions because of the use of oxygen gas and hydrogen chloride gas. In this continuous system, if the concentration of hydrogen chloride is reduced, there is an increase of unwanted by-products and a reduction of the yield of the desired product, so that the introduction of hydrogen chloride gas is indispensable. Also, air cannot be used, in practice, as an oxygen source because much hydrogen chloride accompanies the unreacted gas (waste gas), and the concentration of hydrogen chloride in the chlorination reaction liquid decreases. The equipment and cost of recovering hydrogen chloride from the unreacted gas are substantial items of expense. It is necessary, therefore, to use oxygen gas for practical industrial operation of the process in order to reduce the volume of the unreacted gas.
In this prior art, the reaction of aniline is illustrated by the following reaction equations. EQU (C.sub.6 H.sub.5 NH.sub.2.HCl).sub.2.CuCl.sub.2 + 4CuCl.sub.2 .fwdarw.(ClC.sub.6 H.sub.4 NH.sub.2.HCl).sub.2.CuCl.sub.2 + 2HCl + 2Cu.sub.2 Cl.sub.2 ( 1) EQU 2Cu.sub.2 Cl.sub.2 + 4HCl + O.sub.2 .fwdarw. 4CuCl.sub.2 + 2H.sub.2 O (2)
Therefore, it is required to use 1 mole of hydrogen chloride gas per one mole of aniline. An additional quantity of hydrogen chloride gas is needed to maintain the required concentration of hydrogen chloride in the water which is formed in the reaction. Furthermore, the chlorinated aniline is separated from the reaction system as the complex of cupric chloride and chlorinated aniline hydrochloride. Therefore, it is necessary to use more than 2 moles of hydrogen chloride gas per 1 mole of aniline.
Furthermore, in the prior art, the overall reaction rate is not sufficiently fast. The oxidation step of cuprous chloride to cupric chloride is the overall rate determining step, because of the very small solubility of oxygen gas in the reaction solution.